The present work aims at detecting and characterizing the semiquione free radicals and inorganic composition of atmospheric aerosol particles, PM2.5, collected at three different sites (industrial, residential, and traffic dominated) in Taif city, Saudi Arabia, during 2011/2012. PM2.5 particles have been collected on polycarbonate filters in a collection cartridge in a cyclone. The duration of the collection was 24 h at an air flow of 3 L min(-1). The average PM2.5 mass concentration was 37 +/- 22 mu g/m(3), 57 +/- 22 mu g/m(3) and 50 +/- 31 mu g/m(3) at the residential, industrial and traffic sites, respectively. These values are higher than the target value for air quality standards of European Union (25 mu g/m(3)). Energy dispersive X-ray fluorescence (EDXRF) and electron paramagnetic resonance (EPR) spectroscopic techniques were chosen for sample analysis. Fourteen elements were quantified by EDXRF: Si, S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Rb, Sr and Pb. The measured concentrations of the potentially hazardous trace elements As, Cu, Sb, Cr, Mn, Ni and Pb were either below the detection limit or below the limits defined by international guidelines and national standards of ambient air quality. Furthermore, electron paramagnetic resonance (EPR) technique was used for testing and identifying paramagnetic species in the PM2.5 particles. The EPR spectra of PM2.5 from the three sites have a g-value in the range from 2.0033 to 2.235 which is characteristic for semiquinone radicals. The PM2.5 samples showed three EPR signals indicate the presence of semiquinone radical anion. (C) 2014 Elsevier Ltd. All rights reserved.